1. Field
This patent specification relates to a semiconductor device and, more particularly, to a self-pulsating semiconductor laser device incorporating a saturable absorbing layer having controlled band gap energy and carrier lifetime, and being capable of providing self-pulsation for use in, for example, optical disk systems.
2. Description
Semiconductor lasers incorporating AlGaInP alloy layers are promising devices for use in, for example, optical disk systems for shorter wavelength light emissions. In the optical disk applications of the laser diodes, optical feedback can be a source of excessive noise. One way to avoid the noise is to modulate the laser output at a high pulse repetition rate. This can be achieved by adding a modulator at the cost of complicating the circuit. Alternately, one may utilize to induce self-pulsations by including a saturable absorber in the cavity.
As an example, a self-pulsating laser device is disclosed in Japanese Laid-Open Patent Application 9-283840. Referring to FIG. 4, the laser device is fabricated on an n-GaAs substrate 1 having the direction normal to the surface thereof misoriented by approximately 10° from the direction normal to the (100) plane toward the <011> direction, with the following layers contiguously formed thereon in the order recited:                an n-GaAs buffer layer 2,        an n-AlGaInP cladding layer 3,        an active region 4 of a multi-quantum well (MQW) structure including AlGaInP and GaInP layers,        a p-AlGaInP first cladding layer 5,        a p-GaInP saturable absorbing layer 6, and        a p-AlGaInP second cladding layer 7.        
Further, a ridge stripe is formed of the top portion of the p-AlGaInP second cladding layer 7 extending longitudinally to an optical cavity. A contact layer 8 is then formed on top of the ridge stripe portion of the second cladding layer 7. Subsequently, a current confinement layer 9 is formed flanking the both sides of the second cladding layer 7 and the contact layer 8, and a p-GaAs capping layer 10 is further formed on both the contact layer 8 and the current confinement layer 9.
A positive electrode 11 was subsequently formed on the capping layer 10, while a negative electrode 12 was formed on the rear side of the substrate 1.
Although the saturable absorbing layer 6 and the active layers 4 have the same GaInP composition in this disclosure, the band gap energy of the former has been made smaller by 80 meV than the latter. This is achieved by rendering the crystalline property to be ordered for the former 6, while to be disordered for the latter 4. With the thus prepared structure, the saturable absorbing layer 6 is able to absorb light emissions from the active layer efficiently and the light absorption can be made saturated, to thereby be able to achieve stable self-pulsations.
As another example, a further self-pulsating laser device is disclosed in Japanese Laid-Open Patent Application 9-331098. Referring to FIG. 5, the laser device is fabricated on an n-GaAs substrate 21 with the following layers contiguously formed thereon in the order recited:                an n-GaAs buffer layer 22,        an n-AlGaInP cladding layer 23,        an active layer 24,        a p-AlGaInP cladding layer 25,        a p-AlGaInP saturable absorbing layer 26,        a p-AlGaInP cladding layer 27,        a p-GaInP hetero-buffer layer 28, and        a p-GaAs capping layer 29.        
Subsequently, portions of the p-GaInP hetero-buffer layer 28 and the p-GaAs capping layer 29 are removed to be formed thereto an n-GaAs current confinement layer 30, and a p-GaAs capping layer 31 is further formed on top of both the p-GaAs capping layer 29 and the n-GaAs current confinement layer 30.
A positive electrode 32 is subsequently formed on the p-GaAs capping layer 31, while a negative electrode 33 is formed on the rear side of the substrate 21.
In this disclosure, the saturable absorbing layer 26 is doped with a minute amount of oxygen (i.e., 3×1017 cm−3) in addition to p-type dopants, Zn, with the concentration of 3×1018 cm−3. 
With this oxygen doping, the number of non-radiative recombination centers increases. This results in the expense of minority carriers and also the decrease in carrier life time, to thereby be able to achieve stable self-pulsations.
The pulsating characteristics are known to be affected strongly by the structure parameters and operating temperatures, and several shortcomings have been realized in known laser devices described above.
For example, in the device shown in FIG. 4, the saturable absorbing layer 6 is formed to be in an ordered state and additionally added with the level of dopants of as high as 2×1018 cm−3. Since the dopants tends to diffuse because of this high doping level, the ordered state is gradually transformed to a disordered state. With progress of the disordered structure in the saturable absorbing layer, the difference in the band gap energy from the active layer decreases, to thereby cause failures in stable self-pulsations.
Also, in the device shown in FIG. 5, the saturable absorbing layer 26 is formed being doped with a minute amount of oxygen in order to decrease carrier life time. However, crystalline properties of the Al containing alloy systems such as AlGaInP tend to be degraded by residual oxygen atoms from the reaction chamber and source materials, because of the highly reactive nature of Al.
This degradation may be obviated by increasing the Al content in the AlGaInP saturable absorbing layer 26. Specifically, since the rate of the oxygen doping increases with the increase in the Al content, carrier life time may decrease with a relatively small supplying rate of oxygen. However, in order the band edge wavelength of the saturable absorbing layer 26 be approximately the same as the active layer, this gives rise to another problem, in that the saturable absorbing layer has to be formed including an increased amount of In and thereby accommodating excessive compressive strains.